The astonishing fungus versus the super survivor thistle

Thistles are nasty weeds on farms around the world, including New Zealand farms and the Californian thistle (Cirsium arvense) is the worst of the bunch. It’s such a nuisance that, much like the month-old yogurt pottle languishing at the back of the work fridge, nobody wants to associate themselves with it. The Americans call it the Canadian thistle whilst elsewhere it is called the Californian thistle. But, in reality, it originates from the Mediterranean region and was brought to North America by Europeans. Its original name, creeping thistle is much more appropriate as it is the only thistle with a creeping root system.

Californian thistles. Picture by Wendy Kentjens

This root system is part of what makes the Cirsium arvense thistle so pervasive. Thriving in grasslands, crop-growing lands, and wastelands in temperate areas of the world, it competes well with pretty much all grass and crop plants. It quickly nabs space where more useful plants could grow. And, like some kind of B-movie sci-fi monster, it can even break up those sneaky creeping roots in pieces and grow a new plant from every piece!

Livestock dislike the prickles and don’t eat them, preferring to munch away at other plants, leaving plenty of room for these thistles to grow. Spraying it won’t help for long, as the sprays don’t kill the roots, so the thistles will just regrow, and regrow, and regrow. These thistles are experts at survival!

To beat this formidable opponent we need to stack our arsenal and integrate a suite of control methods. For example, we could let an insect munch away at them, such as the green thistle tortoise beetle, and mow the tops off a few times a year to deplete their energy.

Recently, Lincoln University researchers unearthed what could be a secret weapon: a presumably European fungus Puccinia punctiformis. This is a rust fungus, which naturally occurs on the Californian thistle and potentially could be militarised against it. The spores of this fungus have the same colour as iron oxide, which forms on iron, hence the name rust.

Even at a high altitude of 800 meters such as on this Rakaia Gorge farm (Cleardale station) the Californian thistles grow happily. Picture by Wendy Kentjens

Puccinia punctiformis is also very good at survival, so it is a good match for the super survivor thistle. Even more exciting is that it also infects the roots. No other control method has been able to deal with the roots, making this exciting news!

Naturally, our thistle super-villian isn’t going to make things that easy. The fungus and thistle evolved together, so the thistle knows its enemy pretty intimately and is able to defend itself quite effectively. Thus, to use the fungus to kill the thistle, the balance needs to be shifted in favour of the fungus.

To do that, we first need to understand better how they interact. That is what the first research in this area focused on. They looked at thistles that were visibly sick and those that were infected but did not appear to be suffering. What they found was that the visibly sick plants began the season growing from infected roots. Conversely, the plants that did not look sick despite the presence of the fungus were most likely colonised by spores of the fungus that landed on the leaves.

It’s all in the roots. If the fungus spreads up from here, the thistle is beaten!

A Californian thistle leaf infected by Puccinia punctiformis. Picture by Janet Graham. License CC by 2.0

So the challenge is to shift this balance, but how?

Wounding the thistles would make it easier for the fungus to move into the thistle and down to the roots. On flat land, this could be done easily by mowing, but this won’t work on hills that are too steep for mowers. How do we wound them on steeper land?

The idea might sound a bit crazy, but it wouldn’t be the first time a crazy idea turned out to be a good one. The idea is to train cows to snack on thistles! ‘Wait what?… Training a cow?!’ you might think.

Yes indeed!

Thistles are highly nutritious, but cows haven’t discovered this yet as, like many of our own food aversions, texture is an issue. We can feed the cows the soft, young tips of thistles and mix it with something the cow already is familiar with and likes. Then it becomes a treat for cows and, once they discover it grows in paddocks, they will be looking for it.

“The fungus makes the plant smell sweet and the new growth softer” says researcher Seona Casonato. The softness of the leaves makes them extra yummy! An experienced cow could nibble off those soft bits and leave the prickly parts. This would create an open wound and an easy entry point for the fungus to tunnel in deeper. Then it just needs to travel through the plant to get to the roots.

A cow licking her lips. Is she thinking about those delicious thistles? Picture by Ulrike Leone from Pixabay. License: Free for commercial use

But there is still a lot more to learn before we start to train cows. Thistles didn’t get this far without a fair few tricks up their sleeves, and we would be wise to get a clearer picture of the rust fungus-thistle interaction. For example, does the thistle have ways to prevent the fungus from spreading through the plant? Or will it break off pieces of root to make sure those will not be infected and can grow into healthy plants? And how will it react to being eaten by a cow?

But we of the science world don’t shy from a challenge. And if it means ending the reign of Californian thistle, and welcoming the new rule of nibbling cows – well, that’s pretty persuasive. The Lincoln University team is hard at work to make this happen – watch this space!

Wendy Kentjens is a postgraduate student at Lincoln University. She wrote this article as part of her assessment in ECOL608: Research Methods in Ecology.

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